It's easy to forget to turn off a bike light (or any other safety lights you use), and when you forget, the battery goes dead pretty quick. Blogger Miceuz was fed up with killing their battery, so they modified the light to turn off automatically.

The auto-shutoff switch is remarkably simple, and just uses a spring and a small circuit. When the bike is moving, the spring bounces and keeps the light on. When the bike stops moving for about a minute, the spring stops bouncing and the light turns off. It's a low-tech solution, but works well. Check out Miceuz's post for the full how to.